1. Field of the Invention
This invention relates to an adhesive in two parts. The two parts are mixed together immediately prior to use. Part One is a stable aqueous alkaline phenolic resole resin solution containing a methylene donor. Part Two is a stable resorcinolic precondensate resin having a shortage of formaldehyde which optionally contains a catalyst for curing the resole resin. Each part exhibits viscosity stability of the respective resin similar to that without the methylene donor or catalyst until the two parts are mixed. On mixing, the methylene donor of Part One reacts with the resorcinolic resin of Part Two and the catalyst in Part Two reacts with the phenolic resole resin of Part One to cause adhesive gelation and cure of the resins. The adhesive is particularly useful for the manufacture of lignocellulosic panels such as strandboard.
2. Description of the Prior Art
Aqueous alkaline monohydroxylic phenolic resole resins are used in the structural board industry as durable binders to produce panel products intended for limited exterior use. Such resins have long been plagued with some drawbacks including a relatively long curing time and sensitivity to high moisture levels. Adhesive improvements in recent years have shortened curing times and reduced moisture sensitivity, but advances in these areas are still desirable. The introduction of polymeric isocyanate (polymeric methylene diisocyanate, or PMDI) as a binder for structural boards offers a new performance benchmark for phenolic resole resins. PMDI is a very fast curing adhesive that is relatively unaffected by wood moisture in the normal range encountered in a panel manufacturing plant. PMDI is very expensive, though, and questions are being raised about worker health concerns in plants using the adhesive.
The use of phenolic resole resin curing catalysts are known, e.g., such as those described in U.S. Pat. No. 4,831,067 of May 16, 1989 to Lemon et al; and U.S. Pat. No. 4,961,795 of Oct. 9, 1990 to Detlefsen and Phillips. The above mentioned Detlefsen and Phillips patent is also concerned with methods for accelerating the cure of phenolic resole resins in the manufacture of lignocellulosic panels. The following patents are also concerned with methods for accelerating the cure of phenolic resole resins: U.S. Pat. No. 4,373,062 of Feb. 8, 1983 to Brown which uses a phenol-formaldehyde resole resin to which resorcinol is added and subsequently cured with formaldehyde; and U.S. Pat. No. 4,977,231 of Dec. 11, 1990 to McVay which discusses shortcomings of phenolic resole resins including the use of ester functional curing agents such as propylene carbonate. McVay uses certain carbamate catalysts to solve the problem of slow cure.
Kirk-Othmer, Encyclopedia of Chemical Technology, Third Edition, Volume 13, pages 58 and 59 describes resorcinol adhesives as room temperature curing and water resistant and states that resorcinol-formaldehyde and resorcinol modified phenol-formaldehyde resins are used to laminate wood. Also, Knop and Pilato, Phenolic Resins, 1985, page 13, states that resorcinol or resorcinol-formaldehyde prepolymers can be used as accelerating compounds for curing phenolic resins and that the addition of 3% to 10% of such compounds permits a shorter cure cycle in the manufacture of particle board.
Resorcinol is very expensive in relation to phenol and mixtures of resorcinol resin in monohydroxylic phenolic resole resins are not efficient from the standpoint of the increase in performance in relation to the amount of resorcinol resin needed. Additionally, for curing of such resins prepared from both resorcinol and phenol, the prior art added formaldehyde or paraformaldehyde to the mixture.
Resorcinolic resins will not cure without the addition of an alkylene donor such as a methylene donor, e.g., formaldehyde or hexamethylenetetramine (hexamine). However, with such a methylene donor, resorcinolic resins have the ability to cure at room temperature and at elevated moisture contents. Resorcinolic resins are made with a deficiency of formaldehyde so as to be stable until the time of use, at which time sufficient formaldehyde or other methylene donor is used to cure the resin.
Another source of methylene donors for resorcinolic resins is a class of chemicals called oxazolidines. These can be reaction products of formaldehyde and certain amino alcohols derived from nitroparaffins. The oxazolidines react with resorcinolic resin solutions which lack formaldehyde, in the manner of formaldehyde, without actually releasing formaldehyde to the solution or to the atmosphere. The use of oxazolidine to catalyze resorcinolic resins, including phenol-resorcinol resins is shown in the following U.S. Pat. No. 3,256,137 of Jun. 14, 1966 to Danielson; U.S. Pat. No. 3,281,310 of Oct. 25, 1966 to Danielson; U.S. Pat. No. 3,698,983 of Oct. 17, 1972 to Bryant; U.S. Pat. No. 3,705,832 of Dec. 12, 1972 to Stephan; and U.S. Pat. No. 3,517,082 of Jun. 23, 1970 to Cockerham. Technical Review, TR No. 4 of the Angus Chemical company also describes oxazolidines as useful curing additives of phenol-formaldehyde and one stage phenolic resins.
In this invention, Part One, of the two-part adhesive of this invention comprising the methylene donor and the alkaline monohydroxylic phenolic resole resin are stable. Part Two which comprises the resorcinolic resin and optionally the catalyst for the resole resin of Part One is also stable. On mixing of the two parts the methylene donor of Part One catalyzes the resorcinolic resin of Part two and when Part Two also contains a catalyst for the resole resin such catalyst catalyzes the resole resin of Part One. The mixture of the Part One and Part Two components, i.e. the adhesive, is applied to lignocellulosic material in the manufacture of composition board which is eventually pressed and heated to cure the adhesive.